The present invention relates generally to a control system for a vehicular suspension, such as automotive vehicle suspension, which is variable of the damping characteristics depending upon vehicle driving condition. More particularly, the invention relates to a vehicular suspension control system which varies damping characteristics of the suspension system depending upon a road surface condition and vehicle speed.
Various uses of road preview sensors have been proposed and developed. For example, SAE Technical Paper Series Nos. 630750 and 800520, respectively published on October, 1968 and February, 1980 show road preview sensors for use in suspension systems for obtaining optimum riding comfort and drivability. In addition, Japanese Patent First Publication No. 57-172808, published on Oct. 23, 1982 discloses a vehicle height control system which includes a sensor which detects rough road conditions and adjusts the vehicle height level depending upon road surface conditions. A vehicle height or level sensor is employed in the disclosed vehicle height control system for monitoring the relative displacement between the vehicle body and wheel axle. The output of the vehicle level sensor is compared with a reference level, which serves as a rough road criterion, and adjusts the vehicle height according to the result of judgement of the road surface conditions.
In another example, Japanese Patent First Publication No. 58-30542, published on Feb. 23, 1983, discloses a variable damping force shock absorber with damping characteristics varying in accordance with vehicle driving conditions. In the disclosed system, the magnitude of relative displacement between the vehicle body and wheel axle is measured and a vehicle height variation indicative signal is derived from the measured displacement and the instantaneous vehicle speed. The vehicle height variation indicative signal value is compared with a reference value which serves as a staff suspension criterion for adjustment of the damping characteristics of the shock absorber in accordance therewith.
On the other hand, Published Japanese Patent Application (Tokkai) Sho. 56-42739 discloses a suspension control system which controls the damping force produced in response to rolling forces depending upon vehicle speed and steering adjustments. The damping force is so controlled as to be HIGH when the vehicle speed is higher than a set speed and the steering adjustments through greater than a predetermined angle are performed. In addition, the owner of the present application has already disclosed a suspension control system in Published Japanese Utility Model Application (Jikkai) Sho. 56-147107. In this system, the damping force produced by a suspension shock absorber is adjusted between HIGH and LOW levels depending upon the vehicle speed, the magnitude of steering adjustments and the angular velocity of steering adjustments.
In considering suspension control with respect to roughness of the road surface condition for controlling damping characteristics between HARD and SOFT, HARD/SOFT suspension control criteria would be better to be varied depending upon the vehicle running speed. For instance, as usually experienced, assuming running on the same road over the same uneven, greater magnitude of vibration may be produced as increasing of vehicle speed. Therefore, when the vehicle is running at relatively high speed, harder damping force would be preferred for running over an uneven which may not cause significant instability if the vehicle goes at sufficiently low speed. Alternatively, when harder damping characteristics is selected even in low vehicle speed, rough ride feeling will be given for the passangers to degrade riding comfort. Therefore, in road surface condition dependent suspension control may be required to vary sensitivity of roughness of the road surface depending upon the vehicle speed.
The present invention is intended to provide a suspension control system which is variable of the sensibility of roughness of the road surface depending on the vehicle speed, and thus provide comfortable vehicle suspension with sufficient drivability.